1. Field of Invention
This invention relates to a process whereby high conversions to acrylonitrile can be obtained at unusually high weight hourly space velocities. More particularly, it relates to a process wherein an entrained ammoxidation catalyst and a gaseous mixture comprising propylene, ammonia, and oxygen are passed through a reaction zone while controlling the superficial linear gas velocity and solids feed rate to achieve a state of fast fluidization.
2. Description of the Prior Art
Acrylonitrile can be prepared by passing a gaseous mixture comprising propylene, ammonia and air over a mixed-oxide catalyst at a temperature between 375.degree. C and 525.degree. C. The reaction is highly exothermic. In a few cases manufacturers have used a multitubular fixed-bed reactor cooled by molten salts; but, usually, a fluidized bed reactor is preferred to facilitate heat removal and maintain a uniform reaction temperature. U.S. Pat. No. Re. 27,718 to Sennewald, et al and U.S. Pat. Nos. 3,230,246, 3,427,343 and 3,472,892 to Callahan et al are illustrative of the state of the art in fluidized-bed reactor design and operation, as applied to propylene ammoxidation.
In the fluidized bed processes of the prior art, the reactant gases are passed upward through a bed of suitably-sized catalyst particles at a velocity sufficiently high to buoy the particles and impact to them a violently turbulent fluid-like motion, but not so high as to sweep the bed out of the reactor. A stable bed is maintained which has a distinct surface resembling a boiling liquid. The carry-over of catalyst particles in the reactor effluent is small. Catalyst particles in the 10 to 150 micron size range are preferred for optimum fluidization. With particles of this size, superficial gas velocities between about 0.5 centimeters per second and about 100 centimeters per second are generally required to achieve a stable fluidized bed.
Though the use of the prior art fluidized bed processes is advantageous from a heat transfer standpoint, it is recognized that there are some inherent disadvantages. For example, it is difficult to achieve complete propylene conversion in a single-stage fluidized bed reactor because a certain amount of the gas residing in the rising bubbles tends to pass through the bed without contacting the catalyst. Furthermore, the back-mixing of reactant and product gases is appreciable and encourages secondary reactions which reduce the selectivity for acrylonitrile. A multi-stage reactor such as described in the patents to Callahan et al minimizes these problems, but is more costly to build and operate. A disadvantage of both the single-stage and multi-stage processes is that gas velocities and, thus, throughput, are limited by the need to maintain a stable bed. As a result, the prior art processes are unable to take full advantage of a highactivity catalyst, such as disclosed, for example, in our pending U.S. Pat. Applications Ser. Nos. 645,418 and 645,419, now U.S. Pat. Nos. 4,045,373 and 4,040,983, respectively.